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Not sure if this would be the correct area for this. But I recently just made a 2.5 gallon batch of a stout. And I had a pre-boil gravity of 1.050 with 3.5 gallons of water. And by the end of the boil I had my 2.5 gallons, but I forgot to take a gravity reading before I pitched my yeast.

Is there any general sort of estimation of gravity increase from boil off? Would anyone like to take a guess at what my gravity would have been?

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Just multiply your measured preboil gravity (in 'points') by the ratio of the volumes. But this assumes all the liquid gets into the fermenter.

So:
50 * 3.5/2.5 = 70. So assuming no losses your gravity should be around 1.070. You will have a very small loss due to wort absorbed by the hops (or any other lost/spilled wort) so it might be tiny bit lower than that but not much. I'd guess you were at 1.068-1.069. Again, assuming all the wort except for those tiny losses mentioned made it into the fermenter.

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